Hot melt adhesives typically exist as solid masses at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing of a variety of disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, sanitary napkins, pantyshields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven products. Other diversified applications have involved paper products, packaging materials, tapes and labels. In these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate. A second substrate is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesive is the lack of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating costly processes associated with liquid carrier removal.
For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film through a slot die by using piston or gear pump equipment. In these cases, the substrate is brought into intimate contact with a hot die under pressure. The temperature of the die must be maintained well above the melting point of the adhesive, typically in the range of 150 to 200.degree. C. For some applications, particularly for manufacturing nonwoven articles, bonding of delicate and heat sensitive substrates, such as thin gauge polyethylene film, is often involved. Direct contact between the substrate and the die, in these cases, must be avoided to prevent the substrate from burning or distorting. Several application methods have been developed through which a hot melt adhesive can be spray coated with the aid of compressed air onto the substrate from a distance. These techniques include spiral spray, and various forms of melt-blown methods. Direct contact between the coating head and the substrate is thus eliminated. All the coating techniques mentioned above are well known to those skilled in the art and commercial equipment is readily available.
The indirect coating techniques, however, pose stringent requirements on hot melt adhesives. The viscosity of the adhesive must be sufficiently low, usually in the range of 2,000 to 30,000 cP, preferably in the range of 2,000 to 15,000 cP, at the application temperature. Many other physical factors, especially the rheological properties of the adhesive, come into play in determining the sprayability of a hot melt adhesive. The majority of commercial hot melt products do not lend themselves to spray applications. There are no widely accepted theoretical models or guidelines to predict sprayability and it must be determined empirically with application equipment.
Adhesives based on EPR are known in the art. Adhesives based on EPR alone lack sufficient cohesion and thus prior art EPR adhesives often rely on chemical curing to improve cohesive strength and bonding characteristics. Due to the presence of curing agents, these adhesives are not suitable for applications with conventional hot melt coating equipment. The curing agents may cause premature curing or crosslinking of the adhesive inside the coater and render the machine useless. Furthermore, most prior art adhesives are either solvent based or in the form of pre-formed tapes. They can not be considered as hot melt adhesives.
Tynan et al U.S. Pat. No. 5,798,175 discloses pressure sensitive adhesive (PSA) compositions based on a natural rubber or synthetic rubber and an amorphous polyolefin blend comprising EPR, hydrogenated polyisoprene and atactic polypropylene (APP). The adhesives are prepared by compounding the above-mentioned polymers, a compatible tackifier and an organic solvent. The adhesives are thus solvent based and not hot melts.
Davis et al U.S. Pat. No. 5,612,144 and European Patent Application EPO 714963A2 disclose an adhesive tape composition comprising a polymer blend of at least one EPDM rubber, or preferably three different grades of EPDM rubbers in equal amounts, and another polymer selected from polyisoprene, polybutadiene, EPR, and mixtures thereof. The tape composition further includes at least one tackifier and a sulfur curing package. The composition has extremely high viscosity and contains curing agents, and therefore, can not be considered as hot melt adhesive in a conventional sense.
Davis et al European Patent Application 0672737A1 discloses an adhesive tape composition comprising at least one crosslinkable EPDM, one or more compatible tackifiers, a filler, a softener and a curing system for the rubber polymer. The inclusion of curing agents and unusually high viscosity make the composition unsuitable for hot melt applications.
Planthottam et al U.S. Pat. No. 5,618,883 discloses a hot melt pressure sensitive adhesive composition comprising a tackified EPR rubber modified with a copolymer of styrene-ethylene-butylene-styrene block copolymer and/or styrene-ethylene-propylene-styrene block copolymer.
McEbrath Jr. et al U.S. Pat. No. 5,010,119 discloses a curable adhesive composition comprising an elastomer grafted with a C.sub.3 -C.sub.10 unsaturated mono- or poly-carboxylic acid or anhydride, a tackifier and a crosslinking agent.
Tape compositions have also been disclosed in U.S. Pat. Nos. 4,379,114, 4,404,056 and 5,242,727. These compositions usually contain an EPDM, or a blend of EPDM with other elastomers including butyl rubber, polyisoprene, halogenated butyl rubber and styrene-butadiene rubber, a tackifier, a curing system and other optional ingredients such as plasticizers and fillers and the like.
It is apparent that all the above prior art adhesive compositions are based on EPDM or EPR rubber, either alone or in combination with other type of rubbers. It is well known to those skilled in the art that all the rubbers noted hereinabove are amorphous polymers. The definition of amorphous polymers and its contrast to crystalline polymers can be found in most polymer chemistry textbooks such as Polymers, Chemistry and Physics of Modern Materials by J.M.G. Cowie, 2nd Edition, Blackie Academic and Professionals.
These prior art compositions rely on curing or chemical crosslinking to acquire good cohesive strength and good adhesion characteristics. When uncrosslinked, adhesives based on EPR and EPDM show poor cohesive strength and poor bond strength. The presence of curing agents, and solvent in some prior art compositions, practically made them useless for hot melt applications since the curing agents may cause gelling or crosslinking of the adhesive before they can be applied to a substrate. Such gelling or crosslinking can take place in any component of the coating equipment that involves storage and transport of the molten adhesive mass to the coating head, thereby plugging the equipment and causing severe damage to the coater itself. Further, viscosities of the prior art adhesives are usually very high and beyond the capability of conventional hot melt coating equipment.